The present invention relates to the simultaneous presentation of two video signals wherein one video signal is designated a master signal to provide sync pulses for the second video signal, and more particularly to an apparatus and its method for continuing the presentation of the second video when the master video signal is lost.
With the present development of the generation of various video signals in a television format, it becomes desirable to display the generated video signal simultaneously with a second video signal, either live or recorded. The second video signal may be a computer generated graphics display which may include alphanumeric characters.
Systems are known for superimposing symbols on a video signal for display on a television monitor. In many of these systems, characters to be displayed, such as closed captioned characters, are transmitted with the TV signal during the blanking portion of the signal.
In other systems, digital signals, such as channel numbers or time-of-day displays, are superimposed over the video display as desired. Character generators are also known which overlay captions on a television signal.
Systems are also known in which computer generated graphics signals are overlaid over a TV video signal. Some of these systems employ phase locked loop circuits for locking the clock of the computer onto the TV video signal. Others are known in which the computer clock is the master which controls the display of the TV video signal. U.S. Pat. No. 4,425,581 issued Jan. 10, 1984 to Schweppe et al. discloses a system in which a computer generated video signal is overlaid on an NTSC video signal by use of horizontal and vertical pulse processor circuits to generate a horizontal start signal and a vertical reset signal. Separate column, line and row registers are used which are under the control of the horizontal start and vertical reset signals for enabling individual scan lines of the computer video signal to commence as desired;
Other patents of interest are:
U.S. Pat. No. 3,499,979 issued Mar. 10, 1970;
U.S. Pat. No. 3,584,142 issued June 8, 1971;
U.S. Pat. No. 3,731,282 issued May 1, 1973;
U.S. Pat. No. 3,891,792 issued June 24, 1975;
U.S. Pat. No. 3,898,644 issued Aug. 5, 1975;
U.S. Pat. No. 4,040,088 issued Aug. 2, 1977;
U.S. Pat. No. 4,051,532 issued Sept. 27, 1977;
U.S. Pat. No. 4,117,511 issued Sept. 26, 1978;
U.S. Pat. No. 4,305,131 issued Dec. 8, 1981;
U.S. Pat. No. 4,396,944 issued Aug. 2, 1983; and
U.S. Pat. No. 4,475,164 issued Oct. 2, 1984.